1. Field of the Invention
The present invention relates to a magnetic testing method and a magnetic testing apparatus which can detect flaws existing in a material to be tested and extending in various directions with high precision by using a rotating magnetic field.
2. Description of the Related Art
Conventionally, as a method of detecting a flaw existing in a material to be tested such as a steel plate, a steel pipe or tube or the like in a nondestructive manner, there have been known magnetic testing methods such as an eddy current testing method, a magnetic flux leakage testing method and the like. The eddy current testing method is a testing method utilizing a fact that an eddy current induced by applying an alternating magnetic field to a material to be tested is disturbed by the flaw. Further, the magnetic flux leakage testing method is a testing method utilizing a fact that in the case that a magnetic field is applied to a material to be tested made of a magnetic body so as to magnetize, if a flaw blocking the magnetic flux generated in the material to be tested exists, the magnetic flux leaks to a surface space at a position where the flaw exists.
In the magnetic testing method, in general, an amplitude of a flaw signal to be detected (a signal obtained from the position where the flaw exists, in testing signals detected by a predetermined detection sensor) becomes maximum, in the case that a direction of the applied magnetic field forms a particular angle with respect to a direction in which the flaw extends. For example, the amplitude of the flaw signal in the magnetic flux leakage testing method becomes maximum in the case that the direction of the applied magnetic field (the direction of the magnetic flux in the material to be tested) is orthogonal to the flaw extending direction, and is lowered in accordance with that the direction of the magnetic field deflects from the direction which is orthogonal to the flaw extending direction.
Accordingly, in order to detect (in order to obtain a detectable amplitude of a flaw signal) whatever direction the flaw extends, there has been proposed a magnetic testing method of applying a rotating magnetic field in which a direction of the magnetic field changes hour by hour to the material to be tested, and detecting the flaws extending in the various directions, based on a testing signal generated by the rotating magnetic field (for example, refer to Japanese Unexamined Patent Publication No. 2002-131285).
In order to generate the rotating magnetic field mentioned above, for example, an exciting coil as shown in FIG. 1 is used. In other words, an exciting coil 10 shown in FIG. 1 is provided with two exciting coils (an X direction exciting coil 1 and a Y direction exciting coil 2) arranged in such a manner that winding directions of lead wires are orthogonal to each other (accordingly, generated magnetic fields are orthogonal to each other), and center positions coincide with each other. Further, a resultant magnetic field of the magnetic fields generated in the exciting coils 1 and 2 rotates 360 degrees around the center positions of the exciting coils 1 and 2 (an angle φ shown in FIG. 1 changes between 0 and 360 degrees), by shifting a phase of an alternating exciting current applied to the exciting coils 1 and 2 by 90 degrees (for example, applying a cosine wave exciting current to the X direction exciting coil 1, and applying a sine wave exciting current to the Y direction exciting coil 2). Accordingly, it is possible to detect the flaws extending in various directions (an angle θ shown in FIG. 1 is between 0 and 360 degrees).
In the meantime, in general, in the case that a signal to be detected (a flaw signal in the case of the magnetic testing method) has a specific frequency component with respect to a signal constituted by various frequency components including a noise, it is often the case that a synchronous detection is used for extracting a signal having the specific frequency component.
In the conventional magnetic testing method which does not utilize the rotating magnetic field, the flaw signal is synchronized with the alternating exciting current. Accordingly, it is possible to extract a flaw signal from a testing signal at a high S/N ratio by synchronously detecting the testing signal by using the exciting current as a reference signal, and extracting a signal which is synchronized with the exciting current. Further, the alternating current signal extracted by the synchronous detection is generally smoothened by a low-pass filter, in order to make a ratio (S/N ratio) between the flaw signal and the noise generated in a random order without being synchronized with the exciting current higher. Preferably, the alternating signal extracted by the synchronous detection is smoothened per unit region corresponding to about two or three cycles of the reference signal (the exciting current), by regulating a time constant of the low-pass filter.
Further, in the eddy current testing method, a phase analysis method is generally used as a method for improving a flaw detection performance by using the signal obtained by synchronously detecting the testing signal. In this phase analysis method, an X signal is set to a signal obtained by synchronously detecting the testing signal using the reference signal, and a Y signal is set to a signal obtained by delaying the phase of the reference signal by 90 degrees so as to synchronously detect the testing signal. Further, the method measures how long the phase of the testing signal is delayed with respect to the reference signal, by setting the X signal to an X-axis component, setting the Y signal to a Y-axis component, and displaying the signal by vector on a two-dimensional plane of an X-Y coordinate system (a locus of a vector leading end is referred to as a Lissajous waveform). For example, in the case of synchronously detecting the testing signal having the same phase as the reference signal, it is possible to obtain the Lissajous waveform extending along the X axis as shown in FIG. 2A because of no phase delay. More specifically, in the case of the flaw signal, since the phase is inverted by 180 degrees when the detection sensor passes just above the flaw, it is possible to obtain the Lissajous waveform extending along a direction 0 degrees (a positive direction of the X axis) and a direction 180 degrees (a negative direction of the X axis). In the same manner, with regard to the testing signal in which the phase is delayed by 45 degrees with respect to the reference signal, it is possible to obtain the Lissajous waveform extending along a direction 45 degrees and a direction 225 degrees as shown in FIG. 2B. Further, with regard to the testing signal in which the phase is delayed by 90 degrees, it is possible to obtain the Lissajous waveform extending along a direction 90 degrees and a direction 270 degrees as shown in FIG. 2C.
At this point, it is a rare case that a phase of the flaw signal detected by the magnetic testing method (that is, a signal caused by a turbulence of the eddy current by the flaw, and a signal corresponding to the leakage magnetic flux by the flaw) becomes absolutely identical to a phase of a liftoff varying noise (a fluctuation of the testing signal generated in the case of varying a clearance between the detection sensor and the material to be tested) corresponding to one kind of main noises at a time of testing, and they generally have a phase difference. FIGS. 3A and 3B are schematic views of the Lissajous waveform indicating the fact that the flaw signal and the liftoff varying noise have the phase difference. As shown in FIG. 3A, it is general that a phase φd of the flaw signal is different from a phase φl of the liftoff varying noise. Further, as shown in FIG. 3A, if the amplitude of the flaw signal is set to Ad, and the amplitude of the liftoff varying noise is set to A1, the S/N ratio (=Ad/Al) becomes about 1.5 in this example. However, as shown in FIG. 3B, since the S/N ratio (=Sd/Sl) becomes larger than 10 in this example, by rotating the X-Y coordinate system in such a manner that the liftoff varying noise extends along the X axis, and setting a signal component in a direction Y′-axis in the X′-Y′ coordinate system after the rotation to the testing signal, the S/N ratio is widely improved in comparison with the case that the S/N ratio is evaluated by the amplitude (FIG. 3A). As mentioned above, there can be expected that it is possible to suppress an influence of the liftoff varying noise with respect to the flaw detection performance, by applying the phase analysis method.
Further, the phase analysis method includes a method of evaluating only an amplitude of a signal component having a specific phase in the Lissajous waveform, and excluding the amplitude of the signal component having the other phase from the subject to be evaluated, in addition to the method of rotating the X-Y coordinate system of the Lissajous waveform as mentioned above.
However, the conventional magnetic testing method utilizing the rotating magnetic field has the following problems due to using the exciting current having the single frequency.
(1) Since it is not possible to sufficiently obtain the effect of the synchronous detection, there is a risk that the flaw detection performance (S/N ratio) is lowered.
(2) It is not possible to estimate an angle information of the flaw (what direction the flaw extends in).
(3) It is not possible to use the phase analysis method which is general as a method of improving the flaw detection performance (S/N ratio) in the eddy current testing method.
(4) It is not possible to accurately evaluate a continuity of the flaw.
Therefore, in accordance with the conventional magnetic testing method utilizing the rotating magnetic field, it is possible to conceptually detect the flaws extending in the various directions, however, it cannot be said that the flaw detection performance is practically sufficient. Further, since it is not possible to estimate the angle information of the flaw, it is hard to determine a cause by which the flaw is generated. A description will be specifically given below of the problems (1) to (4).
As mentioned above, in the magnetic testing method, in general, the amplitude of the detected flaw signal becomes maximum in the case that the direction of the applied magnetic field forms a specific angle with respect to the direction in which the flaw extends. In this case, it is assumed that the amplitude of the flaw signal comes to 0 if an angle of shift of the direction of the magnetic field from the direction in which the amplitude of the flaw signal becomes maximum cuts across ±α degree. In the conventional magnetic testing method utilizing the rotating magnetic field generated by the exciting current having the single frequency using the exciting coil 10 as shown in FIG. 1, since the direction of the magnetic field is rotated by 360 degrees during one cycle of the exciting current, the flaw signal appears (the amplitude of the flaw signal becomes larger than 0) under the assumption mentioned above only in a specific range in one cycle of the exciting current (a range in which the direction of the magnetic field between −α degree and +α degree can be obtained based on the direction in which the amplitude of the flaw signal becomes maximum).
In this case, it is assumed that two kinds of flaws A and B (an angle θ (see FIG. 1) of a flaw A equals to 20 degrees, and an angle θ of a flaw B equals to 70 degrees) having different extending directions exist in the material to be tested, and the angle α equals to 20 degrees. As mentioned above, since the amplitude of the flaw signal in the magnetic flux leakage testing method becomes maximum in the case that the direction of the applied magnetic field is orthogonal to the flaw extending direction, the flaw signal of the flaw A becomes maximum in the case that the direction φ (see FIG. 1) of the magnetic field satisfies a relation φ=20 degrees+90 degrees+180 degrees×n (n is an integral number) under the assumption mentioned above, and the amplitude comes to 0 if it goes beyond the range φ±20 degrees. In the same manner, the flaw signal of the flaw B becomes maximum in the case that the direction φ of the magnetic field satisfies a relation φ=70 degrees+90 degrees+180 degrees×n (n is an integral number), and the amplitude comes to 0 if it goes beyond the range φ±20 degrees.
FIG. 4 is a graph showing a time sequence relation between the exciting current waveform and the flaw signal waveform, under the assumption mentioned above. Further, FIGS. 5A and 5B are graphs each showing a flaw signal waveform after synchronously detecting the testing signal including the flaw signal by using the exciting current as the reference signal, and smoothening the flaw signal extracted by the synchronous detection per unit region corresponding to two cycles of the reference signal. FIG. 5A shows the flaw signal waveform of the flaw A, and FIG. 5B shows the flaw signal waveform of the flaw B. In this case, in FIGS. 4, 5A and 5B, an illustration of the noise waveform included in the testing signal is omitted.
In the case of synchronously detecting the testing signal, the synchronous detection uses the exciting current applied to the X direction exciting coil 1 shown in FIG. 1, or the exciting current applied to the Y direction exciting coil 2 as the reference signal, however, as can be seen from FIG. 4, the flaw signals obtained from the flaws A and B are shorter in the cycle than any exciting current. In other words, since the cycle of the flaw signal does not coincide with the cycle of the reference signal, it is not possible to sufficiently obtain the effect of the synchronous detection (the effect of extracting the flaw signal from the testing signal at the high S/N ratio), and there is a risk that the flaw detection performance is lowered (the problem (1) mentioned above).
Further, in the case of smoothening the flaw signal extracted by the synchronous detection per unit region corresponding to two cycles of the reference signal, as shown in FIGS. 5A and 5B, the phase information of the flaw signal (the angle information of the flaw) after smoothening is lost, and the flaw signals after smoothening come to a similar direct current signal waveform in both of flaws A and B. In other words, the angle information of the flaw cannot be estimated (the problem (2) mentioned above).
Further, since the phase information of the flaw signal after smoothening is lost as mentioned above, and it is not possible to specify what position the flaw signal exists in one cycle of the exciting current, it is necessary to always evaluate based on the ratio between the amplitude of the flaw signal and the amplitude of the noise as mentioned above with reference to FIG. 3A, at a time of evaluating the flaw detection performance (S/N ratio). In other words, the general phase analysis method cannot be used as the method of improving the flaw detection performance (the problem (3) mentioned above).
Further, there has been conventionally proposed a method of comprehending a two-dimensional distribution state of the flaw based on a testing image, and evaluating a continuity of the flaw, for the purpose of accurately evaluating the continuity of the flaw so as to improve a flaw detection precision. Specifically, this method forms a testing image (a gray image or a color image) by imaging the testing signal including the flaw signal or by imaging a signal obtained by binarizing the testing signal by a predetermined threshold value. And this method comprehends the two-dimensional distribution state of the flaw and evaluates the continuity of the flaw, by visually observing the testing image, or applying an image process using an appropriate image processing filter or the like to the testing image. This is because there is a case that a plurality of flaws (group defects) extending in the same direction are recognized as one flaw, and a length of a whole of the group defects is recognized as an evaluation index of a harmfulness, in addition to the evaluation of a depth and a length of the individual flaw segmented and detected at a time of evaluating the harmfulness of the flaw. This index is provided for evaluating the harmfulness higher in comparison with the flaw which is actually segmented, in the case where the flaws are actually constructed by one continuous flaw, even if the flaws are segmented into a plurality of pieces and detected. Accordingly, it is important to accurately evaluate the length of the whole of the group defects, that is, the continuity of the flaw.
However, in the conventional magnetic testing method utilizing the rotating magnetic field, since it is not possible to estimate the angle information of the flaw as mentioned above, it is necessary to form the testing image based on only the amplitude information of the testing signal. Accordingly, with regard to the flaw which is detected in the segmented manner, for example, due to a partial small depth of the flaw in spite that the flaw is actually constituted by a single continuous flaw, and is displayed in the segmented manner in the testing image, it is hard to accurately evaluate the continuity of the flaw (the problem (4) mentioned above). Particularly, in the case that position resolution of the testing image cannot help lowering (common case from a restriction of a detection efficiency in the line feeding the material to be tested at a high speed), since it is not possible to set a scanning interval of a detection sensor which is sufficiently small in comparison with a dimension of the flaw, it is not possible to obtain an accurate information what direction the flaw extends, based on the testing image itself. Accordingly, it is hard to accurately evaluate the continuity of the flaw. A description will be more specifically given below with reference to FIGS. 6 to 8.
As shown in FIG. 6, there is assumed a case that two flaws A and B and a noise source N exist in a material to be tested S. Further, there is assumed that pixel groups exist within a testing image obtained by scanning the detection sensor on the material S (a testing image imaging the signal obtained by binarizing the testing signal by a predetermined threshold value), as shown in FIG. 7. The pixel groups are discretized in correspondence to an A/D conversion speed, a scanning speed or the like of the detection sensor, and correspond to a candidate flaw position in the material S. In other words, the pixel groups are constituted by four pixel groups a1 to a4 corresponding to the flaw A, two pixel groups b1 and b2 corresponding to the flaw B, and a pixel n corresponding to the noise source N.
In the case of evaluating the continuity of the flaw with respect to the testing image shown in FIG. 7, since the testing image is formed only based on the amplitude information of the testing signal, the continuity cannot help being evaluated only based on the distribution state of the pixel groups corresponding to the candidate flaw position. Accordingly, there is a risk that the pixel groups a1 to a4 and b1 are erroneously evaluated as one flaw A, and the pixel groups b2 and n are erroneously evaluated as one flaw B, as shown in FIG. 8, in correspondence to the structure of the image processing filter or the like for evaluating the continuity of the flaw. In other words, there is a risk that the pixel n corresponding to the noise source N is erroneously recognized as the flaw, as well as the length of the flaw A is evaluated to be larger than the actual length and the length of the flaw B is evaluated to be smaller than the actual length. Accordingly, there is a risk that the harmfulness of the flaw cannot be accurately evaluated.